Central and peripheral coding of joint position by descending γ-static commands and muscle spindle afferents

The purpose of this study is to investigate plausible central and peripheral coding strategies of joint angle by descending γ_s commands and proprioceptive afferents in humans. Experimental evidence in both human and animal has shown that the firing rate of Ia afferents was linearly correlated to joint angle. Yet, experiments did not elucidate how Ia encoding of joint angle would be affected by α-γ co-activation, and how the peripheral neuromuscular system is informed of central representation of joint configuration. This study is aimed at addressing these issues using a realistic virtual arm (VA) model. In simulated experiments, elbow and shoulder angles of the VA are moved to different angular positions. The γ_s commands to muscles remain constant, or are adjusted with joint angle in linear and nonlinear fashions. The Ia afferents of muscles are evaluated in each case of γ_s modulation. Results show that Ia firing rates of mono-articular muscles can be fine-tuned to the characteristics of experimental recordings, while γ_s commands are nonlinearly modulated with joint angle. This suggests that γ_s commands could serve as the output of central encoding of joint information for posture and movement, and Ia afferents could be used to decode joint angle information faithfully.